The architectural determinants of skeletal muscle function, and how they can be used to optimize our rehabilitation protocols

Transcription

1 The architectural determinants of skeletal muscle function, and how they can be used to optimize our rehabilitation protocols Tim Butterfield PhD ATC FACSM Departments of Rehabilitation Sciences and Physiology Center for Muscle Biology College of Health Sciences Division of Athletic Training University of Kentucky Marjorie A. King Research to Reality Presentation January 10, 2016

2 Return to Play? Adapted from CL Brockett et al. Medicine and Science in Sports & Exercise (2004)

3 Clinical Translational bench to bedside 1. See a problem clinically 2. Delve into the literature 3. Design a model 4. Translate to human

4 Muscle Strain Injury Account for 70% of cases in sports med Garrett, 1990 During a 16 year period collegiate athletes suffered 21,784 muscletendon strains of the lower extremity (LE) Agel et. al., 2007 Re-injury rates: AFL 34% Brockett et al., 2004 After 1 year of return to sports, re-injury rate was 70% Sherry and Best, 2004

5 RF Moment RF Moment Cyclists vs Runners R= R= 0.66 RF Length RF Length Adapted from Herzog et al., MSSE 1991

6 Chicken or Egg? Athletes select sports that fit their muscle function OR Muscle can adapt to performance demands of the sport

7 When you hear muscle think sarcomere Purves et al., Life: The Science of Biology, 4th Edition

8 Sarcomere structure Alternating light/dark bands sarcomere Dark: A band myosin containing force producing cross bridges Light I band - actin only Z-disk anchor for actin

9 Sarcomere structure Arranged in parallel sarcomere Arranged in series Sarcomere number is predictive of function

10 Architectural Parameters Physiologic Cross-Sectional Area (PCSA) Isometric force generation

11 Advantages? Parallel Sarcomere Number Influences PCSA Increased isometric force PCSA F i

12 Architectural Parameters Physiologic Cross-Sectional Area (PCSA) force generation FL/ML ratio fibre velocity fibre extensibility

13 Advantages? Serial Sarcomere Number Dynamic Contractile velocity (concentric) L F V

14 Force / Power [normalized] 1.0 Force Power 0.5 Velocity [normalized]

15 Force / Power [normalized] 1.0 Force Power 0.5 Velocity [normalized]

16 2.0 Force 1.0 Eccentric 0.5 Concentric Velocity

17 Advantages? Serial Sarcomere Number Dynamic Contractile velocity Static Flatter appearance of Force-Length Relationship (FLR) Increased extensibility (eccentric)

18 The FLR Force [Normalized] Ascending Plateau Sarcomere Length [µm] Descending

19 Force [Normalized] Plateau Descending Ascending Muscle Length [Normalized]

20 Example: Muscle: Sarcomere number: 5000 Optimal length for sarcomere force production: 2.0µm (2x10-6 m) At what length does the muscle produce optimal force? 10 mm

21 Example: Muscle: Sarcomere number: 5000 Optimal length for sarcomere force production: 2.0µm (2x10-6 m) At what length does the muscle produce optimal force? What if you add 1000 sarcomeres in series? 12mm

22 Muscle fibre architecture Fibre arrangement within muscle is a major determinant of 1. functional properties 2. contractile properties parallel unipennate bipennate fusiform Fibre arrangement is the greatest determinant of muscle function!

23 Muscle Architecture tendon aponeurosis

24 Muscle Architecture - Examples Leiber, Skeletal Muscle Structure, Function & Plasticity, 2002.

25 Muscle Architecture - FL/ML Based on architecture, what is the major (primary) function of each muscle? velocity or force?

26 Muscle Architecture In general, Hamstrings & dorsiflexors: high FL/ML ratio low PCSA Functionally designed for: high excursions high velocities Quads & plantarflexors: low FL/ML ratio high PCSA Functionally designed for: force production

27 Can we influence these properties? Tabary et al High correlation between joint angle and sarcomere number Passive stretch = increased serial sarcomere number McComas, 1996

28 Williams et al., days immobilization and stimulation 11% increase in length (~2000 sarcomeres in series)

29 What happens during immobilization? Alteration of muscle architecture = atrophy 1. Loss of sarcomeres in series 2. Loss of sarcomeres in parallel

30 Exercise Effect on Sarcomere Number Plateau Force [Normalized] Descending Ascending Muscle Length [Normalized]

31 Joint Moment [Nm] FLR Post Chronic Exercise (short) R 2 = 0.99 R 2 = ±1.7 o Tibiotarsal Joint Angle [degrees]

32 Joint Torque [Nm] FLR post chronic exercise (long) R 2 = 0.99 R 2 = ±1.9 o Tibiotarsal Joint Angle [degrees]

33 % Increase in Joint Torque 140 Increase in Isometric Torque * * * * * * * * * * * * Tibiotarsal Joint Angle [degrees] Butterfield and Herzog Pflugers Archiv, 2006

34 Summary of Results Muscle sarcomere number adapts to eccentric exercise rapidly We can influence the magnitude of the adaptation by altering working range

35 EMG Signal [normalized] Fiber Length [mm] Time [seconds] Butterfield et al., J Appl Physiol, 2005

36 EMG Signal [normalized] Fiber Lenngth [mm] Time [seconds] Butterfield et al., J Appl Physiol, 2005

37 Vastus Intermedius Butterfield et al., J Appl Physiol, 2005

38 Vastus Lateralis Butterfield et al., J Appl Physiol, 2005

39 Summary of Results Muscle responds to exercise by altering sarcomere number Exercise specific Optimizes function for demands Greater starting length during eccentric results in greater sarcomere number addition

40 So what? Adapted from CL Brockett et al. Medicine and Science in Sports & Exercise (2004)

41 Clinical Translation - Strength LeStayo et al., Clin Orthop Rel Res 2009

42 Clinical Translation - Strength

43 Clinical Translation - CAR L Lepley et al., 2015 Knee

44 Clinical Translation - CAR L Lepley et al., 2015 Knee

45 Clinical Translation Length dependence and exercise specific adaptations Be aware of muscle architecture and function Quads vs hamstrings Be aware of demands of sport

46 Eccentric exercise 1. Follow guidelines for any strength and conditioning program with respect to progression

47 Eccentric exercise 1. Follow guidelines for any strength and conditioning program with respect to progression 2. Keep in mind, that peak eccentric force can be up to 2x peak isometric force! I. Must take injury into consideration II. Standing unloaded eccentrics

48 Eccentric exercise 3. Progressively increase the length at which eccentric exercises are applied e.g. Hamstrings: a. Can start with hamstring lowers

49 From CL Brockett et al. Medicine and Science in Sports & Exercise 33(5), (2001)

50 Eccentric exercise 3. Progressively increase the length at which eccentric exercises are applied e.g. Hamstrings: a. Can start with hamstring lowers b. prone knee extensions a. Short length to long c. Progress to sitting knee extensions a. Short length to long

51 Eccentric exercise 4. Progress from slow speeds to high speeds a) Keep the F-V property in mind. Very slow velocity = less tension Moderate to high velocities have little influence on tension!

52 Take Home Message We can influence muscle architecture, and thus alter function Be aware of inherent function of the muscle Force vs excursion Eccentric vs concentric Hamstrings should always be exercised eccentrically Apply exercise prescription to optimize these parameters Pay specific attention two joint muscles to during rehab

53 Thank you!